1. Field of the Invention
The invention generally relates to power amplifiers (PAs) and more specifically to ground partitioning and routing in high-power stages for the purpose of ensuring stable operation.
2. Prior Art
Achieving stable operation of power amplifiers (PAs) over all design corners is a challenging task. The alternating currents (AC) running through the signal path stages of the PAs is very large. This results in large voltage drops on the parasitic inductor paths. A stable operation is crucially linked to a judicious selection of the signal path power and ground lines partitioning.
FIG. 1 shows a typical PA signal path where the ground connections of each gain stage 110, e.g., stages 110-1, 110-2 and 110-3, and cascode transistors 170, e.g., 170-1 and 170-2 runs through specific parasitic series inductances 120, e.g., inductors 120-1, 120-2, 120-3 and 120-4, to the ground plane (reference potential) via inductance 160. Any voltage drop created by the current in one of the back-end stages that has a feedback path toward the inputs of the front-end stages will result in a global feedback loop closed around signal path stages that have large forward gains. This may therefore lead to PA instability if certain conditions for a positive feedback are met. Specifically these include a total loop gain larger than unity, and a total phase shift around the loop equal to 360° or an integer multiple thereof. To avoid the ground feedback loops the common inductance part 160 of the ground connection needs to be as small as possible so that the different gain stages 110 do not interact through the ground connections.
One solution is to use a Kelvin connection of the grounds in which all the stages have a connection to the ground plane effectively resulting in the common inductance 160 being zero. However, this technique requires a large number of ground pins that in most cases are not economical or even not feasible. Separate output stage ground inductance also results in reduced transistor drive voltages due to the negative feedback created by the ground inductance degeneration of the transistor.
Most of the instability of PAs is linked to the way in which the grounds are routed to the common ground plane. The grounds of different signal path stages are directly interconnected via wires that have a finite parasitic inductance. In contrast, the positive supply usually comes through a bias circuit, e.g. bias circuits 140-1, 140-2, and 140-3, which may be in voltage or current mode, which isolates each stage from the rest of the signal path. Furthermore, each stage usually has a local bypass capacitance 130, e.g., capacitors 130-1, 130-2, and 130-3, that helps even further with the supply isolation. This is the reason for having much less supply instability issues in a PA. Such supply instability may happen if the bias circuitry that isolates each signal path from the other stages gets overloaded.
In most PA architectures it is the ground connection that may lead to instability since the ground pins of the different building blocks of the signal path have very little isolation between them, resulting in parasitic feedback loops. The positive supply voltage connections are generally much less sensitive to couplings and parasitic oscillations due to the large isolation provided by the biasing stages for the signal path amplifiers. Both the voltage regulators and current sources, which are the most popular bias circuits, provide very large power supply rejections ratios (PSRR) at low and moderate frequencies and at least some rejection at high frequencies. Saturation of the bias circuits should be avoided since in this case the isolation disappears and instability may occur.
The prior art solution shown in FIG. 1, attempts to break any positive feedback between the output stage ground connections and the different stage path drivers by using a completely segmented ground connection for the output stage. FIG. 2 shows a prior art configuration wherein the input ground (gnd_in) and output ground (gnd_out) are shared. This saves on pins and increased voltage drive, but comes with instability concerns. This at least avoids the ground related instability.
Therefore, in view of the deficiencies of the prior art, it would be advantageous to provide a partitioning solution for the ground of a PA that overcomes at least these deficiencies and taking into account that a PA needs to be built with a cascade of amplifying stages, each such stage having to have a connection to ground.